JP4471911B2 - Connection method of wavelength conversion element - Google Patents

Description

  The present invention relates to a method for connecting wavelength conversion elements, and more particularly to a method for connecting wavelength conversion elements for optically coupling a waveguide formed in the wavelength conversion element and an optical fiber.

  Conventionally, as a wavelength conversion element that converts the wavelength of light, an element that uses a semiconductor optical amplifier, an element that uses four-wave mixing, second harmonic generation, sum frequency generation, and difference frequency generation that are second-order nonlinear optical effects A utilized wavelength conversion element or the like is known (for example, see Patent Document 1). Among them, a quasi phase matching wavelength conversion element (hereinafter referred to as a QPM-LN element) using lithium niobate has a combination of incident light of one or two wavelengths input to the QPM-LN element and a periodicity. By changing the period of a simple domain-inverted structure, converted light having an arbitrary wavelength can be extracted. A light source using a QPM-LN element is expected to put a laser light source into practical use in the visible light region or the mid-infrared region.

  In order to modularize a light source using a QPM-LN element, it is necessary that the pumping light output from one or two semiconductor lasers is incident on the waveguide of the QPM-LN element via an optical fiber. . Therefore, when there is one semiconductor laser, a semiconductor laser having a fiber pigtail for output is prepared, and the fiber pigtail is connected to the waveguide of the QPM-LN element. When there are two semiconductor lasers, two pumping lights are multiplexed by a wavelength multiplexer / demultiplexer including an input / output optical fiber and connected to the waveguide of the QPM-LN element. In each case, the connection between the optical fiber and the waveguide is optically coupled by an optical system using a lens.

  For example, in order to generate mid-infrared light having a wavelength of 2 μm, excitation light output from a semiconductor laser having a wavelength of 1.5 μm and a semiconductor laser having a wavelength of 0.94 μm is combined using a wavelength multiplexer. Wave and enter the waveguide of the QPM-LN element. The mid-infrared light is output by taking out the converted light obtained by the difference frequency generation of the QPM-LN element.

JP 2003-140214 A

  However, the optical circuit of an optical system using a lens has a problem that the number of parts such as a lens part, a holder for YAG welding the lens part, and a metal ferrule is large, and the work process for modularization is complicated. It was. There is also a problem that the price of individual parts such as lens parts is high and it is difficult to reduce the price of the module.

  Further, in the waveguide having the ridge structure of the QPM-LN element, the mid-infrared light oozes out of the waveguide and propagates, so that an adhesive or the like is present around the waveguide at the connection portion between the optical fiber and the waveguide. There was also a problem that it would attenuate when attached.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a method for connecting a wavelength conversion element that can be easily connected with a simple structure by reducing loss at a coupling portion. is there.

In order to achieve the above object, the present invention provides a waveguide and an optical fiber having a ridge structure formed in a quasi phase matching wavelength conversion element having a periodically poled structure. And a concave portion is formed in a base substrate for fixing the wavelength converting element, the waveguide of the wavelength converting element is opposed to the concave portion, and the base substrate is placed below the base substrate. The wavelength conversion element and the base substrate are fixed with an adhesive so that the waveguide does not come into contact with the bottom surface of the recess , with the wavelength conversion element facing upward, and the waveguide of the wavelength conversion element is The first sheath is fixed to the surface opposite to the formed surface, the optical fiber is held in the V-groove formed in the substrate, and the second sheath is fixed to the substrate to support the optical fiber. and, the end face of the waveguide, wherein A first surface and an end face of the optical fiber, the end face and the end face of the second Hire of the substrate is identical to end face and the end face of the first hiring over scan substrate are formed on the same plane The waveguide and the optical fiber are optically coupled by joining a second surface formed on the surface .

The invention described in claim 2 is characterized in that the depth of the recess according to claim 1 is not less than 10 μm and not more than 100 μm.

The invention according to claim 3 is a method of connecting a wavelength conversion element for connecting a waveguide having a ridge structure formed in a quasi phase matching type wavelength conversion element having a periodically poled structure and an optical fiber, A concave portion is formed in a base substrate for fixing the wavelength conversion element, a surface of the wavelength conversion element opposite to a surface on which the waveguide is formed is fixed to a bottom surface of the concave portion, and the wavelength conversion element is a side surface of the concave portion. The wavelength conversion element and the base substrate are fixed so as not to come into contact with each other, and the first sheath is fixed to the base substrate with an adhesive so as to face the waveguide of the wavelength conversion element. The first sheath and the base substrate are kept horizontal, the waveguide and the first sheath are fixed so as not to contact each other, the optical fiber is held in a V-groove formed in the substrate, 2 to the substrate The end faces of and support the said optical fiber, said end surface of the waveguide, a first surface end face and the end face of the first hiring the base substrate is formed on the same plane, said optical fiber, An end face of the substrate and an end face of the second sheath are joined to a second face formed on the same face to optically couple the waveguide and the optical fiber. To do.

According to a fourth aspect of the present invention, the depth of the concave portion according to the third aspect is a depth obtained by adding 10 to 200 μm to the thickness of the wavelength conversion element.

  As described above, according to the present invention, since the gap is provided around the waveguide, the loss in the coupling portion can be reduced, and the waveguide and the optical fiber are directly coupled optically. It becomes possible to connect with a simple structure.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a method for connecting wavelength conversion elements according to Example 1 of the present invention. FIG. 1A is a view of the wavelength conversion element 15 and the optical fiber fixing member 11 to which the optical fiber 14 is fixed as viewed from the side, and FIG. 1B is a view as viewed from the top. FIG. 2 shows a cross section of the optical fiber fixing member 11. The optical fiber fixing member 11 includes a substrate 12 made of quartz or Pyrex (registered trademark) in which a V groove 21 having a depth of 175 μm is formed, and a sheath 13. An optical fiber 14 having an outer diameter of 125 μm is held in the V-groove 21 and fixed with an adhesive, and the sheath 13 is fixed on the substrate 12 with an adhesive. A circular optical fiber 14 is fixed so as to be supported at three points in a triangle formed by the slopes on both sides of the V-groove 21 and the bottom surface of the sheath 13.

  FIG. 3 shows a cross section of the wavelength conversion element 15. In the wavelength conversion element 15, the QPM-LN element 22 is fixed on the base substrate 16 with an adhesive, and the end 17 is fixed on the upper part with the adhesive. The waveguide 23 of the QPM-LN element 22 has a ridge structure. Reference numerals 13 and 17 are members for increasing the bonding area between the wavelength conversion element 16 and the optical fiber fixing member 11.

  With reference to FIG.1 (b), the detail of a connection part is shown. In FIG. 1B, reference numerals 13 and 17 are omitted. The end face of the wavelength conversion element 15 is cut and polished at an angle of 12 degrees. The end surfaces of the opposing optical fiber fixing members 11 are polished at an oblique angle of 18 degrees. This is because the refractive index of the QPM-LN element 22 is 2.1, whereas the refractive index of quartz glass is 1.48, and the direction of the waveguide is in the light propagation direction that is satisfied by Snell's law. This is because of matching.

  By cutting the end face of the waveguide obliquely, the reflected return light at the end face can be reduced. In the case of being cut vertically, if the refractive index of quartz glass is 1.5, 4% (14 dB) of reflected return light is generated from the end face of the optical fiber 14 by Fresnel reflection. In order to obtain a return loss of 40 dB or more, it is necessary to cut the end face obliquely at an angle larger than 8 degrees.

  The structure of the wavelength conversion element 15 will be described with reference to FIG. In the QPM-LN element 22, the surface on which the waveguide 23 is formed and the base substrate 16 subjected to counterboring are opposed to each other, and the end portion of the QPM-LN element 22 and the base substrate 16 are fixed by an adhesive. ing. Since the waveguide 23 has a ridge structure, the mid-infrared light propagating through the waveguide 23 oozes out of the waveguide and propagates. At this time, if an organic substance or an adhesive is present around the waveguide 23, the leaked optical field of mid-infrared light is absorbed by the adhesive and attenuates. Accordingly, the adhesive should not flow into the upper portion of the waveguide 23 of the QPM-LN element 22.

  Therefore, even if the adhesive which joins the QPM-LN element 22 and the base substrate 16 leaks by applying a counterbore process to the base substrate 16, it flows into the recess 23 of the counterbore base substrate 24 and flows into the waveguide 23. Avoid impact. The depth of the base substrate recess 24 is desirably 10 μm or more and 100 μm or less. If the depth is 10 μm or less, it is difficult to reliably maintain a gap with the base substrate 16 when the QPM-LN element 22 is fixed. Further, when the depth is 100 μm or more, there is a possibility that application of the waveguide to the waveguide when the end face of the waveguide is polished. Preferably, the base substrate recess 24 having a depth of 50 μm is formed. The base substrate recess 24 is preferably filled with a gas that does not absorb in the mid-infrared region, such as dry air, rare gas, or dry nitrogen.

FIG. 4 shows a configuration of a wavelength conversion element according to Example 2 of the present invention. The QPM-LN element 34 is fixed to the base substrate 32 subjected to counterboring with an adhesive, with the surface on which the waveguide 35 is formed facing the edge 33. A base substrate recess 36 is formed so that the adhesive does not flow around the waveguide 35. The depth of the base substrate recess 36 is preferably higher than the height of the QPM-LN element 34, and is preferably a depth obtained by adding 10 μm to 200 μm to the thickness of the QPM-LN element 34. If only 10 μm is added, there is a possibility that the QPM-LN element 34 and the heel 33 come into contact with each other. Further, when 200 μm or more is added, there is a possibility that application of the waveguide to the waveguide when the end face of the waveguide is polished. Preferably it is preferable to form the base substrate recess 36 having a depth plus 50μm in the thickness of the QPM-LN element 34. The base substrate recess 36 is preferably filled with a gas that does not absorb in the mid-infrared region, such as dry air, rare gas, or dry nitrogen.

  In the second embodiment, the base substrate recess 36 is formed in the base substrate 32, but the recess 33 is formed in the end 33, and the waveguide 35 of the QPM-LN element 34 is opposed to the end 33 and fixed. Good.

As a member used for the base substrate 32 or the heel 33, a member having a thermal expansion coefficient close to the LN value of 17 × 10 ⁻⁶ (1 / K) is desirable in the sense of reducing the adhesive stress. If the thermal expansion coefficient of extremely different, for example, when a quartz glass (thermal expansion coefficient of <10 × 10 ^-7), thermal strain is generated in the waveguide by the temperature rise or cooling after adhesion, in the worst case May damage the waveguide. Although it is most preferable to use an LN substrate, it is preferable to use a lithium tantalate (hereinafter referred to as LT) substrate (thermal expansion coefficient 15 × 10 ⁻⁶ ), a multicomponent glass (for example, Sumita Optical PFK85, PFK75 glass), or the like. it can.

  The thickness of the base substrate 32 is desirably a thickness obtained by adding 400 μm or more to the thickness of the QPM-LN element 34. If the substrate thickness of the counter-bored base substrate recess 36 is 400 μm or less, the strength of the member is not sufficient, and there is a risk of damage when fixing to a polishing jig for end face polishing. Further, when the thickness of the QPM-LN element 34 is 2 mm or more, the heat conduction of the member is deteriorated, and the temperature control of the QPM-LN element 34 becomes difficult. When the thickness of the QPM-LN element 34 is 500 μm, the thickness of the base substrate 32 is desirably 1.0 mm to 1.5 mm.

FIG. 5 illustrates a method for manufacturing the QPM-LN element according to the third embodiment. In Example 3, ridge waveguide type QPM-LN elements 22 and 34 are fabricated using Zn-doped LiNbO ₃ . First, a Z-cut Zn-doped LiNbO ₃ substrate 41 and a Z-cut Mg-doped LiNbO ₃ substrate 42 on which a periodically poled structure is prepared in advance are prepared. The substrates 41 and 42 are both 3-inch wafers whose surfaces are optically polished and have a thickness of 500 μm.

  After making the surfaces of the substrates 41 and 42 hydrophilic by normal acid cleaning or alkali cleaning, the two substrates are superposed in a clean atmosphere. The superposed substrates are put in an electric furnace and bonded by heat treatment at 500 ° C. for 3 hours (first step). The bonded substrates are void-free, and cracks do not occur even when returned to room temperature. Next, the thickness of the substrate 41 is reduced to 8 μm using a grinding device such as a grinder and a polishing device. After polishing the substrate 41, polishing is performed to obtain a mirror-polished surface (second step).

  Next, the polished thin film substrate is set on a dicing saw, and a ridge waveguide having a core width of 7 μm is manufactured by precision processing using a diamond blade having a particle diameter of 4 microns or less (third step). The grooves 43a and 43b formed by dicing have a depth of about 20 μm. The depth of the grooves 43a and 43b is desirably deeper than the core film thickness, that is, 11 μm. When the film thickness is shallower than the core thickness, the waveguide's equivalent refractive index changes due to slight fluctuations in the groove depth in the direction of travel of the waveguide. Because. A preferable depth of the grooves 43a and 43b is about twice the thickness of the core to 40 μm or less. Processing deeper than 40 μm is not preferable because the mechanical strength of the waveguide itself deteriorates. The manufactured substrates are cut into strips, and the end faces of the waveguide are optically polished, whereby QPM-LN elements 22 and 34 having a length of 50 mm are obtained.

Note that the Z-cut Zn-doped LiNbO ₃ substrate 41 in advance periodically poled structure is formed, may be used and LT substrate. Further, the non-doped LiNbO ₃ substrate and the LT substrate may be used, or the QPM-LN elements 22 and 34 can be manufactured by combining the Z-cut Mg-doped LiNbO ₃ substrate and the LT substrate.

  The thickness of the substrate is not limited to 500 μm, and a substrate having a thickness of 200 μm to 1 mm can be used. When the substrate thickness is 200 μm or less, voids may occur at the bonding interface during bonding due to warpage of the substrate itself. In addition, when a substrate thicker than 1 mm is used, the material cost of the substrate itself becomes high, which increases the manufacturing cost.

  Using the QPM-LN element 34 manufactured in Example 3, the wavelength conversion element 31 of Example 2 is manufactured. A base substrate recess 36 is formed in the base substrate 32 made of PFK85 glass. The thickness of the QPM-LN element 34 is 500 μm, and the depth of the base substrate recess 36 is 550 μm. The QPM-LN element 34 is bonded and fixed to the base substrate recess 36 using a UV effect resin. Next, the adhesive 33 and the base substrate 32 are bonded and fixed using a UV effect resin. The end face of the wavelength conversion element 31 is cut at an angle of 12 degrees and optically polished. At this time, no chipping or the like occurs in the waveguide 35 of the QPM-LN element 34. In order to prevent chipping of the waveguide 35, it is also possible to polish after embedding the upper portion of the waveguide 35 to a length of about 1 mm from the end face using a UV adhesive.

  The position at which the connection loss is minimized after the optical fiber fixing member 11 to which the optical fiber 14 shown in the first embodiment is fixed is prepared, light having a wavelength of 1.5 μm is input to the optical fiber 14 and alignment is performed. Then, the wavelength conversion element 31 and the optical fiber fixing member 11 are bonded and fixed. Using an optical fiber coupler, the light of wavelength 1.54 μm and the light of wavelength 0.94 μm are combined and input to the optical fiber 14 to obtain mid-infrared light of wavelength 2.4 μm with an efficiency of 50% / W. be able to.

  Using the QPM-LN element 22 produced in Example 3, the wavelength conversion element 15 of Example 1 is produced. The base substrate 16, which is an LT substrate, is counterbored to form a base substrate recess 24 having a depth of 50 μm. The surface of the QPM-LN element 22 on which the waveguide 23 is formed and the base substrate recess 24 are arranged so as to face each other, and the end of the QPM-LN element 22 and the base substrate 16 are fixed with a UV effect resin. . Next, the short 17 and the QPM-LN element 22 are bonded and fixed using a UV effect resin. The end face of the wavelength conversion element 15 is cut at an angle of 12 degrees and optically polished. At this time, no chipping or the like occurs in the waveguide 23 of the QPM-LN element 22. In order to prevent chipping of the waveguide 23, it is also possible to polish after embedding the upper portion of the waveguide 23 to a length of about 1 mm from the end face using a UV adhesive.

  The position at which the connection loss is minimized after the optical fiber fixing member 11 to which the optical fiber 14 shown in the first embodiment is fixed is prepared, light having a wavelength of 1.5 μm is input to the optical fiber 14 and alignment is performed. Then, the wavelength conversion element 31 and the optical fiber fixing member 11 are bonded and fixed. Using an optical fiber coupler, the light of wavelength 1.54 μm and the light of wavelength 0.94 μm are combined and input to the optical fiber 14 to obtain mid-infrared light of wavelength 2.4 μm with an efficiency of 50% / W be able to.

It is a block diagram which shows the connection method of the wavelength conversion element concerning Example 1 of this invention. It is sectional drawing which shows the structure of an optical fiber fixing member. It is sectional drawing which shows the structure of a wavelength conversion element. It is sectional drawing which shows the structure of the wavelength conversion element concerning Example 2 of this invention. It is a figure which shows the manufacturing method of the QPM-LN element concerning Example 3 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Optical fiber fixing member 12 Board | substrate 13, 17, 33 Yato 14 Optical fiber 15, 31 Wavelength conversion element 16, 32 Base board 21 V groove | channel 22, 34 QPM-LN element 23, 35 Waveguide

Claims (4)

A method for connecting a wavelength conversion element for connecting an optical fiber and a waveguide having a ridge structure formed in a quasi phase matching wavelength conversion element having a periodically poled structure,
A recess is formed in a base substrate to which the wavelength conversion element is fixed, the waveguide of the wavelength conversion element is opposed to the recess, the base substrate is on the lower side, the wavelength conversion element is on the upper side, and the waveguide The wavelength conversion element and the base substrate are fixed with an adhesive so that they do not come into contact with the bottom surface of the concave portion, and a first mirror is provided on the surface opposite to the surface on which the waveguide of the wavelength conversion element is formed. Fixed,
Holding the optical fiber in a V-groove formed in the substrate, fixing a second sheath to the substrate, and supporting the optical fiber;
An end face of the waveguide, an end face of the base substrate and an end face of the first sheath are formed on the same face , an end face of the optical fiber, an end face of the substrate , and the second face the end face of Hire is by joining a second surface formed on the same plane, the connection method of the wavelength conversion element, which comprises coupling the optical fiber and the waveguide optically.   2. The method for connecting wavelength conversion elements according to claim 1, wherein the depth of the recess is 10 [mu] m to 100 [mu] m. A method for connecting a wavelength conversion element for connecting an optical fiber and a waveguide having a ridge structure formed in a quasi phase matching wavelength conversion element having a periodically poled structure,
A concave portion is formed in a base substrate for fixing the wavelength conversion element, a surface of the wavelength conversion element opposite to a surface on which the waveguide is formed is fixed to a bottom surface of the concave portion, and the wavelength conversion element is a side surface of the concave portion. Fixing the wavelength conversion element and the base substrate so as not to come into contact with
The first sheath is fixed to the base substrate with an adhesive so as to face the waveguide of the wavelength conversion element, and the first sheath and the base substrate are kept horizontal, and the waveguide Fixed so that it does not come into contact with the first
Holding the optical fiber in a V-groove formed in the substrate, fixing a second sheath to the substrate, and supporting the optical fiber;
An end face of the waveguide, an end face of the base substrate and an end face of the first sheath are formed on the same face , an end face of the optical fiber, an end face of the substrate , and the second face the end face of Hire is by joining a second surface formed on the same plane, the connection method of the wavelength conversion element, which comprises coupling the optical fiber and the waveguide optically.   4. The wavelength conversion element connection method according to claim 3, wherein the depth of the concave portion is a depth obtained by adding 10 to 200 [mu] m to the thickness of the wavelength conversion element.

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